The present invention relates to jaw assemblies for use with a chuck and, more particularly, to quick-change jaw assemblies which accurately locate the top jaws to achieve close tolerance workholding following jaw changeover without remachining of such top jaws.
Machining operations such as turning, grinding, milling and inspection require a workpiece to be held accurately with respect to the cutting tool. In many such applications, the design specifications require a repeatable accuracy of at least 0.0001" total indicator reading (TIR). Workpieces may be held with such accuracy by use of what are commonly referred to as "precision chucks." These chucks are typically power actuated, i.e., the jaw assemblies are movable in a radial direction (to grasp and release the workpiece) by means of an actuating mechanism and a source of power such as compressed air. To ensure the required accuracy, the top jaws used to hold the workpiece are typically machined to the required size and configuration once such jaws are secured to the chuck, the chuck being fixed to the machine.
However, as will be recognized by those skilled in the art, it is often necessary to change the top jaws to allow holding of different sized/configured workpieces on that particular machine. Alternatively, the top jaws may be removed from one machine so that such jaws may be installed on another machine to allow an additional machining operation to be performed on the same workpiece. This changing of top jaws is a labor intensive operation in that the holding accuracy previously established on the machine is lost once such jaws are removed from the machine. This accuracy must again be established through careful and labor intensive setup of the top jaws on the new machine, and typically must be followed by at least one machining operation of the top jaws after they are again secured to the chuck. On many machines, including certain automatic machines, the need to remachine the top jaws after changeover is a difficult/expensive operation. Additionally, a particular set of top jaws can only be remachined a limited number of times.
Various companies have attempted to overcome the disadvantages associated with jaw changeover by providing what are now referred to as "quick-change" jaws. These quick-change jaws, which are promoted as providing high accuracy even after jaw changeover, generally include top jaws which cooperate with the underlying master jaws to position such top jaws thereon. These prior art quick-change jaws may approach repositioning accuracies of 0.0001" TIR, but only if each of the top jaws is repositioned on the same master jaw of the same chuck. Accordingly, the top jaws must be labeled when removed from the chuck so as to allow identical repositioning of the jaws on the chuck. An additional drawback associated with many prior art quick-change jaw assemblies is that the design of the assembly allows either external or internal clamping of the workpiece, but not both types of clamping.
One known prior art quick-change jaw assembly designed and commercialized by the assignee herein utilizes a plurality of sub jaws, the sub jaws being mounted to the radially-movable master jaws. Each of the sub jaws includes a ground section connected to the body of such sub jaw by a flexing web. The top jaw is secured to the sub jaw by means of a single, centrally-located screw which engages a threaded hole in the ground section. As the screw is tightened, the ground section is caused to pivot about the web towards the top jaw, thus causing the ground section to positively engage at least one surface of the top jaw thereby locating the top jaw with respect to the sub jaw. This prior art quick-change jaw assembly, although providing repositioning accuracy after jaw changeover, suffers from at least the following disadvantages: 1) Because of the mechanical advantages created by the cooperation of the screw/ground section, overtightening of such screw adversely impacts the tolerances previously established in the top jaw thereby negatively affecting the close tolerance workholding required in a machining operation, particularly when internal clamping of the workpiece is required; 2) The need to utilize sub jaws introduces additional tolerances and costs to the machining operation; and 3) The need to utilize sub jaws results in the top jaws being moved a distance away from the datum plane defined by the face of the chuck thereby magnifying (at the workpiece location) the tolerances found in the components of the chuck and jaw assemblies.
There is therefore a need in the art for a quick-change jaw assembly which provides repositioning accuracies of at least 0.0001" TIR after jaw changeover, even if the top jaws are not repositioned in the identical fashion on the chuck. There is a further need in the art for a quick-change jaw assembly which allows the top jaws to be moved from chuck to chuck while maintaining repositioning accuracies of at least 0.0001" TIR. Finally, there is a need in the art for a quick-change jaw assembly which eliminates the need for a sub jaw assembly, which limits the ability of an operator to adversely impact the tolerances previously established on the top jaw by overtightening of the top jaws to the master jaws and which minimizes the distance between the workpiece and the datum plane defined by the chuck face.